The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications where quality high speed forward projection and backprojection of data is desired (e.g. magnetic resonance and proton emission).
Generally, CT scanners have a defined examination region or scan circle in which a patient or other subject being imaged is disposed. A beam of radiation is transmitted across the examination region from an x-ray source to oppositely disposed radiation detectors. The segment of the beam impinging on a sampled detector defines a ray extending from the source to the sampled detector. The source, or beam of radiation, is rotated around the examination region such that data from a multiplicity of rays crisscrossing the examination region are collected.
The sampled data is typically convolved and backprojected into an image memory commonly described as a two-dimensional array of memory elements. Each memory element stores a CT number indicative of the transmission or attenuation of the rays attributable to a corresponding incremental element within the examination region. The data from each ray which crossed the incremental element of the examination region contributed to the corresponding CT number, i.e., the CT number for each memory element of the resultant image is the sum of contributions from the multiplicity of rays which passed through the corresponding incremental element of the examination region.
Commonly, the x-ray data is transformed into the image representation utilizing filtered backprojection. A family of rays is assembled into a view. Each view is filtered or convolved with a filter function and backprojected into an image memory. Various view geometries have been utilized in this process. In one example, each view is composed of the data corresponding to rays passing parallel to each other through the examination region, such as from a traverse and rotate-type scanner. In a rotating fan beam type scanner in which both the source and detectors rotate, each view is made up of concurrent samplings of the detectors which span the x-ray beam when the x-ray source is in a given position, i.e., a source fan view. The detectors commonly have either an equal linear spacing or an equal annular. Alternately, with stationary detectors and a rotating source, a detector fan view is formed from the rays received by a single detector as the x-ray source passes behind the examination region opposite the detector.
In forward projection, the image data is processed to synthesis each of the multiplicity of views that went through convolution and backprojection to make the image representation.
Various backprojection and forward projection algorithms have been developed. For CT scanners, it is generally advantageous to have a substantially instantaneous display of the resultant CT images. The many millions of computations required renders general purpose computers inappropriately slow for backprojection. As well, various forward projection software routines have been written for general purpose microcomputers. Again, however, the task of breaking the image representations apart into their component views requires many millions of computations. The execution time for such programs, commonly on the order of many minutes to hours, rendered forward projection too slow for many everyday clinical applications.
To obtain the image representations rapidly, the projections are normally performed with dedicated projection hardware. One such forward projection/backprojection processor for CT scanners is described in U.S. Pat. No. 5,008,822 to Brunnett, et al. incorporated herein by reference. However, certain limitations of the described forward projection/backprojection processor regarding speed, efficiency, space consumption, and the like, make it inappropriate for certain applications.
The present invention contemplates a new and improved forward projection/backprojection processor which overcomes the above-referenced problems and others.